1. Field of the Invention
The present invention relates to an optical cable, and more particularly to an optical cable capable of enduring tensile force applied from outside.
2. Description of the Related Art
An optical cable has one or more optical fiber which shows sensitive optical features against an external force, so several protective layers are coated on the optical fiber so that the optical fiber is sufficiently protected from the external force during or after installation of the optical cable.
Though the optical cable has a thick protective layer, a section of the optical fiber is no more than several millimeters with a long length. Thus, if the optical fiber is inserted into the optical cable by a length identical to the optical cable, the optical fiber is apt to be easily cut off or stretched in a longitudinal direction due to the external tension or the change of external temperature caused by the change of seasons, thereby deteriorating optical characteristics of the optical cable.
In order to solve this problem, there is proposed a method for inserting an optical fiber into a metal tube and then surrounding the metal tube with a sheath to make an optical cable, in which the optical fiber has a length somewhat longer than the metal tube. At this time, a length of the optical fiber in excess of the optical fiber is called excess fiber length (EFL). Though the sheath and the metal tube are expanded or contracted due to external tension, the optical fiber in the optical cable is not influenced by the external tension since EFL is prepared.
Seeing conventional techniques for generating EFL, there is a method for spirally twisting an optical fiber to be put into the metal tube, and then surrounding the optical fiber with a metal tube so that the spiral optical fiber has a longer length than the straight metal tube.
As another method, a straight metal tube having wrinkled surface is used. In this method, an optical fiber is inserted into the wrinkled metal tube, which is then surrounded by a sheath to make an optical cable. At this time, a cooling process is executed so that the wrinkled metal tube is shortened when the sheath of the optical cable is thermally contracted, thereby making the length of the optical fiber longer than the contracted optical cable.
However, such conventional methods have problems that the optical cable is significantly affected by characteristics of the optical fiber inserted into the cable as well as material or composition of the metal tube. In particular, in case a plurality of optical fibers are received in an optical cable, EFL formed depending on the number of optical fibers and the size of the optical cable does not have a constant value, so it is not easy to control the processes for obtaining a desired EFL.